This invention relates to improved power connectors for superconducting magnets which connect external power through the pressurized magnet vessel to the coils inside the vessel.
As is well known, a magnet can be made superconductive by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen which slowly boils off, forming a cryogen gas which is vented out of the cryostat. The extreme cold reduces the resistance in the magnet coils to negligible levels, such that when a power source is initially connected to the coil (for a period, for example, of only ten minutes) to introduce a current flow through the coils, the current will continue to flow through the coils due to the negligible resistance at superconducting temperature even after power is removed, thereby maintaining a magnetic field. Superconducting magnets find wide application, for example, in the field of magnetic resonance imaging (hereinafter "MRI").
In a typical superconducting magnet, the cryostat or pressurized helium vessel is positioned within, but spaced from, a vacuum vessel with a significant temperature gradient therebetween. As a result, the shim lead connector assembly and power leads must extend through a wide temperature range from the outside of the vacuum vessel to the interior of the pressurized vessel.
A multi-pin cryostat connector typically extends through the wall of the cryostat for coupling to the plurality of shim or adjusting magnets, commonly with male pins on the exterior and extending away from the cryostat, to enable coupling through a mating female connector. The cryostat connector can be damaged by multiple insertion and retraction operations of the shim lead coupling assembly, particularly since the connectors must be mated by manipulation of the shim lead connector assembly from a distance, typically well in excess of one foot, from the outside of the vacuum vessel.
A power coupling assembly for which the present invention is applicable is disclosed in co-pending U.S. patent application, Ser. No. 07/880,848 filed May 11, 1992 by William S. Stogner and Daniel C. Woods, assigned to the same assignee as the present invention, and hereby incorporated by reference. In such an arrangement, if the connector pins of the cryostat connector are damaged, it is a difficult and costly procedure to replace the cryostat connector since it passes through the pressurized vessel and vacuum vessel. As a result, the pressurized vessel and the vacuum vessel must be cut open, the cryostat connector replaced, and the pressurized vessel and vacuum vessel subsequently reassembled and vacuum space evacuated.
Similarly, a main coil power connection is provided through the pressurized vessel to carry the heavy currents used to energize the main coils. The current to the main magnet coils is far larger than that to the shim magnet coils, such that it is also important to minimize the resistance heating due to that current flow.
However, any solution to these problems must avoid increasing the resistance heating resulting from current flow through the connector assembly, since even a single watt of heating can result in the boiling of 1.4 liters per hour of helium, which is completely unacceptable, since a typical MRI specification limits the helium boil-off to only 0.2 liters per hour. Accordingly, it is important to reduce and minimize the resistance heating resulting from current flow through the MRI power connectors to the superconducting magnets. Also, any solution must insure positive electrical contact, and be practical and useful in the presence of conflicting thermal, electrical and mechanical considerations.